1. Field of the Invention
This invention relates to the preparation of organic polymeric materials with near-zero uniaxial thermal expansion coefficients.
2. Description of the Prior Art
Materials with near-zero thermal expansion coefficients are required for use in optical applications and in other areas of technology wherein high dimensional stability over a specified temperature range is necessary.
Conventional materials which have low linear thermal expansion coefficients (.alpha.), such as metallic alloys and inorganic glasses, have been employed previously. However, metallic alloys such as Invar, which comprises about 64 weight percent iron and 36 weight percent nickel (average .alpha. equal to about 2 .times. 10.sup.-.sup.6 .degree. C.sup.-.sup.1 between zero and 100.degree. C), and Superinvar, which comprises from about 62.5 to 64.0 weight percent iron, 30.5 to 34.0 weight percent nickel and 3.5 to 6.0 weight percent cobalt (.alpha. = 0 to 0.5 .times. 10.sup.-.sup.6 .degree. C.sup.-.sup.1 at 20.degree. C), are both expensive and extremely heavy. In addition, these alloys are difficult to machine. Moreover, glasses such as vitrious SiO.sub.2 (.alpha. = -0.27 .times. 10.sup.-.sup.6 .degree. C.sup.-.sup.1 at -160.degree. C; + 0.1 .times. 10.sup.-.sup.6 .degree. C.sup.-.sup.1 at -60.degree. C; + 0.35 .times. 10.sup.-.sup.6 .degree. C.sup.-.sup.1 at 0.degree. C and +0.50 .times. 10.sup.-.sup.6 .degree. C.sup.-.sup.1 at +60.degree. C) are difficult to machine, heavy and are also very brittle.
Thus, there is a need for a material having high thermal dimensional stability in at least one direction, which is strong, light, easily machined, and inexpensive. Organic polymers are attractive construction materials because of their low densities, low cost, and ease of processing. However, conventional organic polymers do not have high dimensional stability. They creep, i.e., undergo slow and irreversible deformation, under mechanical stress and typically have high thermal expansion coefficients (e.g., linear thermal expansion coefficients of the order of 10.sup.-.sup.4 .degree. C.sup.-.sup.1 for anisotropic linear polymers).
The intrinsic thermal expansion coefficient along the chain direction of a linear polymer is typically small and negative, while the thermal expansion coefficients along orthogonal directions are generally large and positive. However, organic polymers as generally produced are not sufficiently free of regions containing misaligned polymer chains, that is, polymer chains non-parallel with the average chain direction. Thus, the large, positive thermal expansion contribution from such regions overwhelms the much smaller negative chain-direction contribution from highly-aligned chains. Consequently, the observed macroscopic thermal expansion coefficient along the average chain direction, that is, the uniaxial thermal expansion coefficient, is positive. While a higher degree of chain alignment can be obtained for some polymers by drawing processes, morphological changes usually occur at relatively low temperatures which result in irreversible dimensional changes. Consequently, these polymers are not suitable for use as structural elements for applications in which high thermal-dimensional stability is required.